Process for the manufacture of resin-impregnated fiber reinforced structural composites and the product resulting therefrom. More particularly, fiber-wrapped pressure vessels are subjected to pressure and temperature pre-treatment to increase their strength.
It is known to wrap a variety of underlying shapes with fiber so as to form fiber-reinforced plastic composite products, or FRP. The fiber acts as the structural portion wrapped over a normally weak shape. The fibers act in tension when the composite is stressed. One such example is the manufacture of fiber-reinforced pressure vessels by wrapping hollow, substantially non-structural, liners with fibers.
The conventional processes for making fiber-wrapped composites result in composite products which are not as strong under load as they could be.
Conventionally, fibers are so small that they are usually massed into larger groupings before use in making FRP. Typically a multiplicity of fibers, such as upwards of 12000 fibers, are spooled into tows. Multiple tows are passed through heated resin baths containing catalyzed resins prior to being mechanically wrapped onto a liner. The configuration of the winding is dependant upon the speed of rotation of the vessel liner and the rate of travel of the tow-dispensing apparatus. The most common configurations are helical, in which the tows are at a significant angle from the axis of the object being wrapped; circumferential, in which the tows are wound hoopwise around the object; and polar, in which the tows are wrapped in the direction of the longitudinal axis of the object.
The resin is permitted to dry and is then cured. Curing relates to the process by which the resin is allowed to achieve its final chemical state and effect its purpose to provide reinforcement to the liner. Curing or chemical poly-condensation, is the formation of polymers from monomers with the release of water or another simple substance. Curing is usually performed at elevated temperatures however, room temperature may be sufficient for some types of resins.
The cured state is typically where the conventional process ends and the resulting product is pressed into service.
The manufacturing processes of the prior art appear unable to wrap the fibers onto a shape in such a way that all fibers are equally prepared to carry a tensile load, such as when restraining a vessel liner under pressure or a beam under bending. It is hypothesized that fibers within the tows are not all arranged in such a way within the resin so as to be capable of immediately carrying tensile load when pulled. More specifically, it is hypothesized that as the fibers are wrapped, bends are introduced and when the liner is pressurized some of the fibers accept tension and others merely straighten without accepting any significant load.
The result of these inequities or disparities between fibers within the wrap is that structural composites of the prior art are not capable of achieving as strong a wrap as is theoretically possible if all of the fibers shared the load.
The current invention addresses the disparity in tension sharing between fibers as is experienced by the prior art. The solution is achieved by altering the arrangement of the fibers within the resin to effect greater ability to share tensile loads and ultimately increase the strength of the fiber-wrapped shape. This homogenization or equalization of the tensile load between individual fibers is accomplished by causing the fibers to move within the resin in response to load applied to the shape. Under typical operating pressures and temperatures, fibers are substantially immobile within the cured resin. Under the process of the invention, the resin properties are manipulated to permit heightened mobility therein and the fibers are manipulated to permit fibers to achieve a more optimal arrangement.
Generally, a fiber-wrapped shape or liner of a pressure vessel is subjected to elevated temperature and elevated pressure over time. The elevated temperature allows the fibers to become mobile within the resin. The elevated pressure from within the vessel effects a change in the tensile load carried by each of the discrete fibers.
The elevation in temperature of the resin must be sufficient to allow the fibers to be mobile therein and the loading or vessel pressure must be sufficient to stress the fibers and initiate their movement to a new and lower-stress arrangement. Once the resin is cooled to operating temperatures, the multiplicity of fibers share tension load more equally.
In a broad aspect of the invention then, a process for the homogenization of a tension in a plurality of fibers embedded in fiber-reinforced composites comprises the steps of:
raising the temperature of the resin above operating temperatures so as to allow the resin to soften;
manipulating the composite so as to introduce tension into the fibers;
maintaining resin temperature and fiber tension as long as necessary to permit the fibers to move within the resin thereby permitting the tension in the fibers to homogenize; and then
cooling the resin and fiber matrix wherein the multiplicity of fibers are better able to share tensile loads imposed thereon.
Preferably the composite is a fiber-wrapped liner or pressure vessel and the means for introducing tension into the fibers involves pressurizing the liner. Ideally the temperature is elevated to a range in which the fibers become mobile within the resin, but the properties of the resin, the fibers or the shape are not permanently degraded. Further, the tensioning of the fibers is performed without causing failure of the shape or the fibers. An example of a vessel benefiting from such a process is a vessel used for store fuel gases such as natural gas and hydrogen.
More preferably, autofrettage is practiced in series with the homogenizing process or simultaneously therewith and thereby achieving even greater resultant strength.